Triple-picket warm plastic-shell implosions on OMEGA

Abstract

Warm deuterium-gas-filled plastic shells are imploded by direct irradiation from the OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. The pulse shapes contain three pickets that precede a sharp rise to a constant laser intensity at ∼4.5×1014 W/cm2. The in-flight-aspect-ratio (IFAR), a crucial measure of shell instability to nonuniformity growth, is varied in these implosions by changing picket energies and the timing among the pickets. Simulations that include cross-beam energy transfer in addition to inverse bremsstrahlung for the laser-energy deposition models show better agreement with measurements of the neutron bang time and temporally resolved scattered light and therefore more correctly model the shell kinetic energy. It is also shown that target performance improves significantly as IFAR is reduced. Nearly twice the neutron yield is measured for IFAR∼31 compared to IFAR∼60. The ratio of the measured to simulated neutron yield and areal density increases significantly with decreasing IFAR. These implosions unambiguously link target performance to in-flight shell instability attributable to short-wavelength growth and indicate that IFAR≤40 is required to achieve adequate compression at this intensity.